Electricity, Circuits and Magnetism

The term current defines a directed flow of particles we also find in the blood of our body. In order to be able to measure blood flow, one needs electrical appliances that require electricity. The electric current is generated through an electric circuit over which charges are moved through potential differences. The movement of the potential differences also happens in the human body, for example in the sodium potassium ATPase.

00:01
Now that we've discussed electricity and how to discuss quantitativelyelectricity, as well as some of the variables there, we're ready to talk about circuits.
00:10
To contextualize this, remember that we've already discussed all of our mechanicsand we'll be discussing magnetism after we coverhow electricity flows through all the circuits we're going to get into.
00:21
We'll start this by first introducing the two main ingredients of a circuitthat we'll need to remember and know how to deal with.
00:29
Those are resistors. So we'll talk about Ohm's law and resistors nowand then we'll talk about the other ingredientwhich is a capacitor and then finally going to more complicated circuitsand see how to work with resistors and even capacitors in bigger circuits as well.
00:42
So let's start with Ohm's law and what a resistor is.
00:45
In order to introduce this topic, we first need to have an idea abouthow we discuss the different variables that can be applied on a circuitand then we'll discuss Ohm's law which gives us a relationshipbetween the variables that we introduceand then finally we'll talk about resistors themselvesand how to deal with them on a circuit and how to add them together.
01:07
So first let's introduce three variables: the voltage, current and then the resistance.
01:12
The current in a circuit which will be one of the most important quantitiesthat we're trying to find or measure is telling you how much charge per secondis flowing past a given point. So for example in this wire that we have here,we have a wire with current flowing through it.
01:32
That's all these electrons that we've shown moving hereand then they're all flowing past this dotted line,this particular point in our circuit and current is measuring how many electronsare passing this point in a given amount of time in a given second.
01:47
We measure therefore the current exactly as you would expect.
01:50
It's the number of coulombs, the amount of charge passing a given point per secondand we call this coulombs per second in Amp.
01:58
So amps which is symbolized by the letter A is telling us how much current is flowing in a circuitor how may coulombs per second.
02:06
Interestingly, something important to know is that by historical convention,again something of a historical accident,we actually measure current in the positive direction.
02:16
So even though electrons themselves the actual electricity of the electronsflowing through the circuit is the flow of negative chargeswhich is what the electrons are, we define current to be the flow of positive chargewhich is to say the actual protons positive charges are movingis just to say that the flow of positive charge is defined in the opposite directionas the flow of negative charge. And usually, this won't be problem,not something you have to actually worry aboutbut if you do see electrons drawn on the circuit flowingalways keep in mind that the current by definitionis defined in the opposite direction of that electron flow.
02:53
The second quantity that we actually be familiar with is the voltage.
02:57
We've already discussed the voltage in the sense of just electricityand electric fields and the electric potentialbut we're going to talk about voltage in circuitsas the energy per coulomb again except this timewe'll probably more think about the voltage as a pressurein terms of trying to push the electrons to the circuit.
03:15
We'll talk about batteries as well and how these batteries are acting as pumps,sort of pushing the electricity through your circuit.
03:24
So if you see this battery on the bottom here,the way we talk about this battery is that the positive side of the batteryis the side with the longer line. So we represent the battery in this sort of series of lineswhich is in fact representative of the cells, the battery cellsthat are in this cell or battery and we always, by convention,discuss the positive side of the voltage where which pushingthe current in the positive direction as the part of this battery with small line,big line, small line, big line that ends in the bigger lineand so again this battery is a sort of a pump for our circuit.
04:01
It's trying to push the electricity to flow through the circuit.
04:05
Now, when we have a particular voltage flowing and pushingrather a current through a particular circuit,we can ask ourselves the exact same kind of question that we asked with fluid flow.
04:18
When we had fluid flow, we talked about the pressure,how much pressure we were applying to a systemand then ask for a given amount of pressure how much current,how much flow rate as we call it there was going to flow through our circuit.
04:31
And we saw that that dependent on the resistance of the circuitand so in that context, we call it the Circuit Law for the Flowing Systems.
04:41
In this case, we are actually introducing the circuit lawby writing down the resistance as the ratio of the voltage,the amount of pressure we apply per amount of current that we get out.
04:52
The resistance, this quantity that we've just introduced,has units of volts per amp since it is the voltage divided by the current.
05:02
So we call one volt per amp an Ohmand is represented by a capital Greek letter omega as you see here.

About the Lecture

The lecture Electricity, Circuits and Magnetism by Jared Rovny is from the course Circuit Elements.

Included Quiz Questions

A current is a measure of which of the following?

Amount of positive charge moving past a point per unit time

Number of electrons moving past a point per unit time

Number of electrons

Number of protons

Speed of electrons in a circuit

Which of the following are the units of volts?

Energy per charge

Charge per energy

Charge per time

Flow per second

Current per second

Which of the following is an accurate analogy between electricity in circuits and fluid flow?

Current is like flow rate, voltage is like pressure, and resistance is like friction resistance

Current is like pressure, voltage is like flow rate, and resistance is like friction resistance

Current is like volume, voltage is like force, and resistance is like friction resistance

Current is like fluid mass, voltage is like flow rate, and resistance is like friction resistance

Current is like force, voltage is like volume of flow, and resistance is like friction resistance

Author of lecture Electricity, Circuits and Magnetism

Jared Rovny

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